This invention relates generally to methods for enhancing the properties of a polymer and, more particularly, to methods for compounding a nanostructured chemical into a polymer.
It has long been recognized that the morphology of polymers can be controlled to a high degree through variables such as composition, thermodynamics and processing conditions. (F. W. Billmeyer, Jr., Textbook of Polymer Science 3rd Edition, Wiley and Sons, New York, 1984.) It is similarly known that various sizes and shapes of fillers (e.g. calcium carbonate, silica, carbon black etc.) can be inserted or compounded into a polymer to somewhat control both polymer morphology and the resulting physical properties.
In their solid state all polymers (including amorphous, semi-crystalline, crystalline, and rubber, etc.) possess considerable amounts of internal and external free volume (see FIG. 1). The free volume of a polymer has a tremendous impact on its physical properties, since it is within this volume that the dynamic properties (e.g. reptation, translation, rotation, crystallization) of polymer chains primarily operate and in turn influence fundamental physical properties such as density, thermal conductivity, glass transition, melt transition, modulus, relaxation, and stress transfer.
The accessibility of free volume in a polymer system depends greatly on its morphology. As shown in FIG. 2, for example, denser regions and phase separation within a polymer can both increase and decrease the thermodynamic and kinetic access to such areas. Because of its influence on thermodynamic and kinetic properties, polymer morphology is a major factor that limits the ability of conventional fillers from accessing the free volume regions in a polymer system. Additional processing/compounding effort is normally required to force compatibilization between a filler and a polymer system because conventional fillers are physically larger than most polymer dimensions, are chemically dissimilar, and usually are high melting solids.
Prior art in compounding has focussed on incorporating polymer systems with small, low molecular weight molecules (liquids and solids) known as plasticizers or plasticizing agents and with macro, micro and nanoscale particulates of dissimilar composition (e.g. inorganic) to that of the polymer (organic). The function of a plasticizing agent is to aid in the slippage of polymer chains by one another, thus improving the processability and manufacturability of a particular polymer system. Similarly fillers, which have traditionally been composed of fibrous or particulate solids, have been combined with polymers to enhance physical properties such as dimensional stability, impact resistance, tensile and compressive strengths, and thermal stability. (F. W. Billmeyer, Jr., Textbook of Polymer Science 3rd Edition, Wiley and Sons, New York, 1984, Chapter 6, pp 471-472). Unfortunately, where plasticizers are too small to reinforce polymer chains, traditional fillers are too large to reinforce individual polymer chains and segments. As illustrated in FIG. 4, fillers are traditionally utilized to macroscopically reinforce large associated or nearby groups of polymers rather than the individual chains and segments within these polymers.
Nevertheless, it has been calculated that as filler sizes decrease below 50 nm, they would become more resistant to sedimentation and more effective at providing reinforcement to polymer systems. (G. Wypych, Fillers, ChemTech Publishing, Canada, 1993.) The full application of this theoretical knowledge, however, has been thwarted by the lack of a practical source of particulates with monodispersity and diameters below the 50 nm range and especially at or below the 10 nm range. Particularly desirable are particles that are monodisperse or which have controlled and narrow particle size distributions as these are expected to form the most stable dispersions within polymer systems. In addition, these particles would be well below the length scale necessary to scatter light and hence should appear transparent when compounded into plastics.
Recent developments in nanoscience have now enabled the ability to cost effectively manufacture commercial quantities of materials that are best described as nanostructured chemicals due to their specific and precise chemical formula, hybrid (inorganic-organic) chemical composition, and large physical size relative to the size of traditional chemical molecules (0.3-0.5 nm) and relative to larger sized traditional fillers ( greater than 50 nm).
Nanostructured chemicals are best exemplified by those based on low-cost Polyhedral Oligomeric Silsesquioxanes (POSS) and Polyhedral Oligomeric Silicates (POS). FIG. 3 illustrates some representative examples of monodisperse nanostructured chemicals, which are also known as POSS Molecular Silicas.
These systems contain hybrid (i.e., organic-inorganic) compositions in which the internal frameworks are primarily comprised of inorganic silicon-oxygen bonds. The exterior of a nanostructure is covered by both reactive and nonreactive organic functionalities (R), which ensure compatibility and tailorability of the nanostructure with organic polymers. These and other properties of nanostructured chemicals are discussed in detail in U.S. Pat. No. 5,412,053 to Lichtenhan et al., and U.S. Pat. No. 5,484,867 to Lichtenhan et al., both are expressly incorporated herein by reference in their entirety. These nanostructured chemicals are of low density, exhibit excellent inherent fire retardancy, and can range in diameter from 0.5 nm to 50 nm.
Prior art associated with fillers, plasticizers, and polymer morphology has not been able to adequately control polymer chain, coil and segmental motion at a molecular level. Furthermore, the mismatch of chemical potential (e.g., solubility, miscibility, etc.) between hydrocarbon-based polymers and inorganic-based fillers resulted in a high level of heterogeneity in compounded polymers that is akin to oil mixed with water. Therefore, there exists a need for appropriately sized reinforcements for polymer systems with controlled diameters, (nanodimensions), distributions and with tailorable chemical functionality. In addition, it would be desirous to have easily compoundable nanoreinforcements that have chemical potential (misibilities) ranges similar to the various polymer systems.
The present invention describes methods of preparing new polymer compositions by compounding nanostructured chemicals into polymers. The resulting nano-alloyed polymers are wholly useful by themselves or in combination with other polymers or in combination with macroscopic reinforcements such as fiber, clay, glass mineral and other fillers. The nano-alloyed polymers are particularly useful for producing polymeric compositions with desirable physical properties such as adhesion to polymeric, composite and metal surfaces, water repellency, reduced melt viscosity, low dielectric constant, resistance to abrasion and fire, biological compatibility, and optical quality plastics. The preferred compositions presented herein contain two primary material combinations: (1) nanostructured chemicals, nanostructured oligomers, or nanostructured polymers from the chemical classes of polyhedral oligomeric silsesquioxanes, polyhedral oligomeric silicates, polyoxometallates, carboranes, boranes, and polymorphs of carbon; and (2) traditional amorphous polymer systems such as: acrylics, carbonates, epoxies, esters, silicones, etc. or traditional semicrystalline and crystalline polymer systems such as: styrenics, amides, nitrites, olefins, aromatic oxides, aromatic sulfides, esters etc. or ionomers or traditional rubbery polymer systems as derived from hydrocarbons and silicones.
Preferably, the compounding of nanostructured chemicals into polymers is accomplished via blending of the chemicals into the polymers. All types and techniques of blending, including melt blending, dry blending, and solution blending are effective.
In addition, selective incorporation of a nanostructured chemical into a specific region of a polymer can be accomplished by compounding into the polymer a nanostructured chemical with a chemical potential (miscibility) compatible with the chemical potential of the region within the polymer to be alloyed. Because of their chemical nature, nanostructured chemicals can be tailored to show compatibility or incompatibility with nearly all polymer systems. Their physical size in combination with their tailorable compatibility enables nanostructured chemicals to be selectively incorporated into plastics and control the dynamics of coils, blocks, domains, and segments, and subsequently favorably impact a multitude of physical properties. Properties most favorably improved are time dependent mechanical and thermal properties such as heat distortion, creep, compression set, shrinkage, modulus, hardness and abrasion resistance. In addition to mechanical properties, other physical properties are favorably improved, including lower thermal conductivity, improved fire resistance, and improved oxygen permeability.